build without Thunderbird support

[Paul Wellner Bou <paul purecodes org>]

Hi,

I tried to compile beagle without thunderbird support (configure 
--disable-thunderbird) because I don't need the mail index very much (as 
there is no way to open thunderbird mails from another application) and 
the debian sid version of beagle i have crashes with the thunderbird 
backend.

Well, i configured and tried to make beagle:
./Tiles/MailMessage.cs(139,23): error CS0103: The name `Thunderbird' 
does not exist in the context of `Search.Tiles.MailMessage'

I commented the else if block out and it worked. But I don't think that 
this is the wished behaviour. :-)

Regards
Paul.